Optimus Prime Full Body Sketch

I drew a pencil drawing of Optimus Prime's body from Beast Hunters while I was home, though I'll admit it was a pain in the ass to draw. I said I was going to draw him when I had the chance here. Why is his head so small compared to his body??

Reference (minus the Star Saber).

Art (c) Me.
Optimus Prime (c) Transformers & Hasbro.